Keratoscope instruments of the Placido disc type have long been used by ophthmologists for examining the surface curvature of the human eye. A more modern form of such instrument is disclosed in U.S. Pat. No. 3,797,921 issued Mar. 19, 1974 to L. G. Kilmer et al. The size of the Placido disc instrument is determined by the number of concentric light rings, or mires, to be projected onto the patient's eye and the more light rings that are to be projected in the vicinity of the limbus, the greater the diameter of the instrument must be. However the greater this dimension the more it tends to block eye contact between the patient and the ophthalmologist thereby causing certain patients some amount of emotional discomfort.
An improved form of apparatus for mapping the contours of the cornea is shown in Gersten, et al, U.S. Pat. No. 4,863,260 issued Sep. 5, 1989. The apparatus described in the Gersten '260 patent employed a cylindrical form of keratometer containing a plurality of illuminated rings incised along the opaquely coated bore of a conical translucent plastic body. The incised rings of the bore were illuminated by an array of incandescent lamps contained in a lightbox disposed adjacent to the base of the cone. Toward the apex of the cone converging beams from a pair of helium-neon laser guns established an optical reference point on the visual axis of the apparatus. For the precise location of the point of laser beam intersection it was desired to have the beams intersect at an appreciable angle of at least 90 degrees. Achieving this angle of intersection required that the laser guns either be positioned at this angle to the visual axis or that a mirror arrangement be employed. In either case the transverse dimension of the apparatus was increased.
As mentioned above, the illuminated rings of the Gersten et al '260 patent were formed by cutting through the opaque coating of the bore to expose the translucent plastic. Variations in the thickness of the opaque coating and in the sharpness of the tools used to cut through the opaque coating affected the regularity of the edges defining the illuminated rings and consequently the distinctness of the rings' image projected on the target. In addition, the plastic cone exhibited a substantial thermal coefficient of expansion. The array of incandescent lamps required to provide sufficient illumination to the base of the conical body to illuminate the incised rings could generate considerable heat. The expansion of the cone could shift the location of the ring pattern relative to the focal point of the instrument necessitating that the apparatus be recalibrated after warm-up. In addition, if it were desired to substitute a cone having a different ring pattern or different bore diameter a painstaking re-alignment of the laser beam guns and mirrors with respect to the visual axis of the apparatus would be required. Such substitution also would require re-programming of the computer controlling the mapping of the eye contour.